There are many themes in Hamlet that you could consider the main one. Personally, I think that human mortality and death is the main theme. Besides revenge, it is the one topic that remains constant throughout the play. From the very first scene until the very last we are aware of death, and what it does to those around it. In fact, almost everyone dies. An example of Hamlet's wit? There are many throughout the play. If, "Brevity is the soul of Wit" (Act 2 Scene 2) as Polonius says when trying to convince Claudius and Gertrude of Hamlet's insanity, or lovesickness, than Hamlet is certainly not a wit, since he speeches tend to go on forever. Hamlet is given towards philosophy and abstraction. That aside, Hamlet's wit is most obvious in the play he puts on for his mother and Claudius. With the play he uses his careful wit to word it, and then to gauge the reaction of Claudius and Gertrude.
One of the most prominent themes of the play can be found at the beginning of what is perhaps Hamlet's most famous soliloquy. Those five lines read:
To be, or not to be, that is the question:
Whether 't is nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing, end them?
What the character here is wrestling with is the puzzling theme of whether revenge is worth it. What is the point of being alive?, he wonders. If there is a point, should one just absorb life's blows and trudge onward, despite "outrageous fortune"? Or is there peace in avenging wrong-doing?
Hamlet is understandably torn. His father was a good ruler and did not deserve his fate, but what good did his goodness do him? He was murdered by his own brother and his wife turned traitor. On the other hand, his father's life (and ghost) seems to demand retribution.
As for wit, I find Hamlet to be one of Shakespeare's most clever plays in terms of the playfulness of language. One of my favorite lines is "Thus conscience doth make cowards of us all" (3.1.91). Hamlet is saying that a man (or woman) might get away with immoral and/or criminal behavior for awhile, but eventually his/her conscience will catch him/her up. He hopes in particular that the conscience of Claudius will be exposed by Hamlet's plot to catch him up will be revealed by his display of cowardice.